This invention relates to a process for preparing a polymer solution. More particularly, it relates to radical polymerization of unsaturated monomers in a solvent to prepare a polymer solution having a reduced content of the unreacted unsaturated monomers.
Several methods for obtaining a polymer solution having a reduced content of an unreacted unsaturated monomer have been proposed to date. Known processes include (1) feeding nitrogen, etc. to vaporize an unsaturated monomer, (2) simple distillation to distill off an unsaturated monomer together with a solvent, (3) addition of a small amount of a polymerization initiator at the final stage of polymerization to accelerate polymerization of the remaining unsaturated monomer, (4) blowing ozone, etc. into the system at the final stage of polymerization to remove an unreacted unsaturated monomer, (5) addition of a basic substance capable of reacting with an acrylic ester monomer (i.e., unsaturated monomer), such as hydrazine, and (6) irradiation with an ionizing radiation, such as xcex3-rays.
However, any of these known methods cannot be recognized as a method providing a polymer solution having its unsaturated monomer content reduced sufficiently and therefore easy to handle, highly safe, and almost odorless.
JP-A-10-87738 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a continuous polymerization process for obtaining a resin with excellent optical characteristics, in which polymerization is carried out in a system comprising not more than 25% by weight of a solvent, and the reaction mixture is distilled while withdrawing the bottom of a distillation column to remove the impurities, etc. by-produced in the polymerization. However, the polymer solution prepared by this method contains a large amount of unreacted monomer and therefore it is difficult for use in the fields where weight is: put on safety, such as the medical field and the cosmetic field.
An object of the present invention is to provide a process for preparing a polymer solution which has a sufficiently reduced residual unsaturated monomer content and therefore is easy to handle, highly safe, and almost odorless.
As a result of intensive studies, the present inventors have found that a polymer solution meeting the above object can be obtained by limiting the weight ratio of an unsaturated monomer(s) and a solvent to be charged and defining the conversion of an unsaturated monomer that has the lowest conversion of the other unsaturated monomers whose boiling point is lower than that of the solvent. The present invention has been completed based on this finding.
The gist of the invention resides in a process for preparing a polymer solution comprising radically polymerizing a monomer mixture comprising at least one unsaturated monomer in a solvent for the unsaturated monomer(s) and the polymer in the presence of a radical polymerization initiator, whereby at least one unsaturated monomer has a lower boiling point than that of the solvent, and wherein the weight ratio of the total amount of the unsaturated monomers to the solvent is in a range of from 1/9 to 7/3, the polymerization is carried out until the conversion of the unsaturated monomer having the lowest conversion of all the other unsaturated monomers whose boiling point is lower than that of the solvent reaches at least 85%, and, after completion of the polymerization, the unreacted unsaturated monomer(s) is removed under rectification conditions.
The unsaturated monomers which can be used in the invention are not particularly limited. A monomer mixture comprising at least one unsaturated monomer whose boiling point is lower than that of the solvent used can be used. Therefore, the monomer mixture can contain an unsaturated monomer which has a higher boiling point than the solvent as long as the monomer is so polymerizable as to remain unreacted in only a trace amount. Further, the monomer mixture can contain an unsaturated monomer having a molecular weight of not less than 1000, such as a silicone macro-monomer, because such a macro-monomer has no problem of smell nor of safety even if it remains unreacted.
Examples of useful unsaturated monomers include unsaturated carboxylic acids, unsaturated carboxylic acid alkyl esters, N-alkyl-substituted amides of unsaturated carboxylic acids, alkyl vinyl esters, styrene derivatives, and alkyl vinyl ethers.
The unsaturated carboxylic acids include acrylic acid, methacrylic acid, and vinylacetic acid.
The term xe2x80x9calkylxe2x80x9d as used herein means a straight-chain or branched saturated hydrocarbon group which may be substituted with a functional group containing a nitrogen atom, an oxygen atom, etc. The alkyl group preferably contains 1 to 12 carbon atoms, particularly 1 to 8 carbon atom. Specific examples of the alkyl group are methyl, ethyl, propyl, n-butyl, isobutyl, t-butyl, 2-ethylhexyl, hydroxyethyl, and dimethylaminoethyl. The alkyl esters of the unsaturated carboxylic acids include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, t-butyl acrylate, t-butyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, dimethylaminoethyl acrylate, and dimethylaminoethyl methacrylate.
The N-alkyl-substituted amides of the unsaturated carboxylic acids include N-methylacrylamide, N-t-butylacrylamide, and N,N-dimethylacrylamide. The alkyl vinyl esters include vinyl acetate, vinyl propionate, and vinyl butyrate. The styrene derivatives include styrene and xcex1-methylstyrene. The alkyl vinyl ethers include methyl vinyl ether, ethyl vinyl ether, propyl vinyl ether, and butyl vinyl ether.
Of these unsaturated monomers preferred are unsaturated carboxylic acids, such as acrylic acid and methacrylic acid; alkyl esters thereof, such as t-butyl acrylate, t-butyl methacrylate, 2-ethylhexyl acrylate, and 2-ethylhexyl methacrylate; N-alkyl-substituted amides thereof, such as N-methylacrylamide, N-t-butylacrylamide, and N,N-dimethylacrylamide; and styrene.
The solvent which can be used in the present invention is not particularly limited. Solvents generally employed in solution polymerization can be used. Those capable of dissolving all the unsaturated monomers to be polymerized and the produced polymer are preferred. Examples of suitable solvents include alcohols, such as ethanol, isopropanol, t-butanol, 3-methoxy-3-methyl-1-butanol, 3-methyl-1,3-butanediol, and 2-methyl-1,3-propanediol; aliphatic hydrocarbons, such as heptane, isoparaffin, and n-paraffin; aromatic hydrocarbons, such as toluene and xylene; esters, such as ethyl acetate, n-propyl acetate, isopropyl acetate, butyl acetate, and 3-methoxy-3-methylbutyl acetate; ketones, such as methyl ethyl ketone and methyl isobutyl ketone; and cyclic silicone solvents, such as octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane. These solvents can be used either individually or as a mixture of two or more thereof.
Preferred among these solvents are multi-component aliphatic: hydrocarbons such as isoparaffin and n-paraffin. Multi-component aliphatic hydrocarbons having 8 to 18 carbon atoms, such as isoparaffin, are particularly preferred.
As the radical polymerization initiator which can be used in the present invention, any radical polymerization initiator known in the art can be used in the present invention. Suitable initiators include peroxides such as benzoyl peroxide and lauroyl peroxide, and azo compounds such as 2,2xe2x80x2-azobisisobutyronitrile, 2,2xe2x80x2-azobis-2,4-dimethylvaleronitrile, and dimethyl 2,2xe2x80x2-azobisisobutyrate.
The polymerization reaction is usually carried out by heating the reaction system comprising the solvent, the unsaturated monomer or monomers, and the radical polymerization initiator to a necessary temperature. The manner of charging the unsaturated monomers or the initiator is not particularly restricted. For example, the requisite amount may be present from the very beginning of the reaction, or may be added during the reaction either continuously or in portions.
What is essentially required is that the weight ratio of the total amount of the unsaturated monomers to the solvent to be charged is in a range of from 1/9 to 7/3. That weight ratio is preferably 1.5/8.5: to 7/3, more preferably 2/8 to 6/4. If the weight ratio of the unsaturated monomers is lower than 1/9, the polymerization rate is too low, which is not only inefficient but disadvantageous for reduction in residual unsaturated monomer content. If the weight ratio of the unsaturated monomers exceeds 7/3, it is difficult to remove the heat of reaction, and the resulting polymer solution is too viscous for the practical use.
The polymerization temperature is selected appropriately so that the radical polymerization initiator may act satisfactorily. It usually ranges 40 to 120xc2x0 C., preferably 50 to 100xc2x0 C., more preferably 60 to 90xc2x0 C. The reaction time, while dependent on the initiator, the monomers, the reaction temperature, etc., is generally 4 to 24 hours, preferably 6 to 22 hours, more preferably 8 to 20 hours.
In carrying out the present invention it is necessary to select the reaction conditions from among the above-mentioned ranges so that the unsaturated monomer which has a lower conversion than any other unsaturated monomer whose boiling point is lower than that of the solvent may achieve a conversion of at least 85%, preferably 90% or higher. If the conversion of the unsaturated monomer which has the lowest conversion among the unsaturated monomers whose boiling point are lower than that of the solvent is less than 85%, not only is the polymer yield poor, but the efficiency in removing the unreacted unsaturated monomers is poor.
If desired, the polymerization may be carried out in the presence of a commonly employed molecular weight regulator, such as mercapto compounds.
After completion of the reaction, the unreacted unsaturated monomers are removed from the resulting polymer solution by distillation to efficiently provide a polymer solution which is easy to handle, highly safe, and almost odorless.
Distillation for unreacted monomer removal after the polymerization is not particularly restricted as far as rectification conditions are selected. Rectification is usually carried out at a number of theoretical plates of 3 or more, preferably 4 or more, more preferably 5 or more, and a reflux ratio of 0.5 or higher, preferably 0.7 or higher, more preferably 1.0 or higher, thereby to distil off the unreacted unsaturated monomers and the solvent. Where the number of theoretical plates is smaller than 3, or where the reflux ratio is smaller than 0.5, a large quantity of the solvent should be distilled off so as to remove the unreacted unsaturated monomers, which is not efficient.
The amount of the solvent distilled off is preferably not more than 20% by weight, particularly 15% by weight or less, based on the amount of the solvent charged into the polymerization reactor.
In order to carry out the present invention effectively, it is preferred that the concentration of the unreacted unsaturated monomers in the distillate be 70% by weight or lower, particularly 60% by weight or lower. Where the concentration exceeds 70%, polymerization of the unsaturated monomers in the distillate is likely to occur in the distillation column, the refluxing tank, etc., which can cause such problems as clogging.
In the present invention, addition of a polymerization inhibitor to the distillation column, etc. is not particularly required. It is rather preferred not to use any polymerization inhibitor for safety considerations particularly in cases where the polymer obtained is used as a base for cosmetics, and the like.
The steps of polymerization and distillation can be conducted either in separate apparatus or in an apparatus. In the former case, the polymer solution from the polymerization reactor is transferred to a distillation apparatus, where the unreacted unsaturated monomers are distilled off. In the latter case, a reactor equipped with a distillation column is used, in which the polymerization reaction is followed by distillation.
There is thus obtained a polymer solution having an extremely low total concentration of the unreacted unsaturated monomers, e.g., 1000 ppm or less, preferably 500 ppm or less, still preferably 300 ppm or less. The resulting polymer solution is of high safety and almost odorless and therefore suitable for uses where weight is put on safety, such as the medical field and the cosmetic field. The polymer solution can be used either as such or as diluted with other solvents. Where an isoparaffinic solvent has been used in the polymerization, the polymer solution can be used in the form of an oil-in-water emulsion.
The present invention will now be illustrated in more detail by using Examples and Comparative Examples, but it should be understood that the invention is not construed as being limited thereto. Unless otherwise noted, all the percents, parts, and parts per million (ppm) are given by weight.